netdev - Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming calculation

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Date:	Thu, 22 Sep 2011 18:26:48 +0200
From:	Oliver Hartkopp <socketcan@...tkopp.net>
To:	Wolfgang Grandegger <wg@...ndegger.com>
CC:	David Miller <davem@...emloft.net>,
	SocketCAN Core Mailing List <socketcan-core@...ts.berlios.de>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2] candev: allow SJW user setting for bittiming
 calculation

On 09/22/11 15:07, Wolfgang Grandegger wrote:

> Hi Oliver,
> 
> On 09/22/2011 12:28 PM, Oliver Hartkopp wrote:
>> This patch adds support for SJW user settings to not set the synchronization
>> jump width (SJW) to 1 in any case when using the in-kernel bittiming
>> calculation.
>>
>> The ip-tool from iproute2 already supports to pass the user defined SJW
>> value. The given SJW value is sanitized with the controller specific sjw_max
>> and the calculated tseg2 value. As the SJW can have values up to 4 providing
>> this value will lead to the maximum possible SJW automatically. A higher SJW
>> allows higher controller oscillator tolerances.

(..)


> As I already said, I expected "bt->sjw" to be always 0 when
> can_calc_bittiming() is called. But the software somehow allows to
> preset "bt->sjw", which is not intended as the help of the ip utility shows:


What was the technical reason for this 'not intended' that time?

> I actually hesitate to extend can_calc_bittiming(). I suggest using an
> external tool to calculate proper bit-timing parameters and set them
> with "ip link set DEVICE type can tq ...".


Yes, i know the possibility to specify the bittiming via time-quanta (tq).

But i think the in-kernel bittiming calculation is pretty good and solves most
usual cases. For me it's just an additional requirement to specify SJW as
MIN(tseg2, max_sjw) to extend the clock source tolerance - which can be
provided as an additional option to the bittiming calculation without pain
(IMO). If i can 'tune' the sample point, why should i not have any influence
to the synchronization jump width generation then?

And btw. a help text can really be changed easily ;-)

-        [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
+        [ bitrate BITRATE [ sample-point SAMPLE-POINT] [sjw SJW] ] |

Regards,
Oliver
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